1. Field of the Invention
The present invention relates to an optical modulator etc. having a multiple quantum well optical absorption layer between a p-type semiconductor and an n-type semiconductor.
2. Description of Related Art
The electroabsorption optical modulator (EA modulator) is an optical modulator that utilizes the electroabsorption effect that the optical absorption coefficient (hereinafter abbreviated as “absorption coefficient”) of a substance varies depending on the electric field applied to it. More specifically, the EA modulator modulates light by inducing an absorption coefficient variation in a semiconductor absorption waveguide layer by applying an electric field to it. The EA modulator is generally classified into a type using an absorption waveguide layer having a quantum well structure and a type using a bulk semiconductor layer rather than a waveguide layer.
FIGS. 8A and 8B are energy band diagrams of a conventional EA modulator using an absorption waveguide layer having a quantum well structure, and FIG. 8C is an energy band diagram of a conventional EA modulator using a bulk semiconductor layer. In FIGS. 8A to 8C, reference symbol 101 denotes a p-type semiconductor; 102, a multiple quantum well structure; 102a, a one-period quantum well structure within the multiple quantum well 102; 103, an n-type semiconductor; 104, a p-type-semiconductor-side (p-side) barrier layer or an n-type-semiconductor-side (n-side) barrier layer; 105, a single quantum well; and 106, a bulk semiconductor layer.
Conventionally, in optical modulators for modulation systems having modulation rates (bit rates) of 2.5 Gbps and 10 Gbps, measures have been taken to produce good light waveforms with a low degree of chirping by optimizing the absorption layer structure. The chirping (or chirp) herein means a phenomenon that the light frequency varies continuously as a function of time. However, with an increase in the bit rate that is required for optical modulators, producing good light waveforms with a low degree of chirping is now becoming difficult. In particular, there is a problem that no effective measures for reducing the degree, of chirping haven been proposed yet for optical modulators for modulation systems having a bit rate of 40 Gbps.
A description will now be given of how reducing the degree of chirping becomes more difficult as the bit rate that is required for optical modulators increases. FIGS. 9A and 9B show typical modulation signal waveforms of driver circuits for driving a conventional optical modulator. More specifically, FIGS. 9A and 9B show modulation signal waveforms in the case where the bit rate is equal to 10 Gbps and 40 Gbps, respectively. As shown in FIG. 9A, where the bit rate is equal to 10 Gbps, the modulation signal waveform is close to a rectangular one in which the 0level and the 1-level can easily be discriminated from each other as seen from part Pa that is enclosed by a broken line. On the other hand, as shown in FIG. 9B, where the bit rate is equal to 40 Gbps, even with a driver circuit that is optimized for this modulation rate, a waveform that is closer to a rhombic one than to a rectangular one can only be obtained as seen from part Pb that is enclosed by a broken line. Further, as shown in FIG. 9B, the spread σm of the 1-level line is increased, which is a factor of increasing an error ratio of fiber-transmitted codes in optical communication systems. As described above, in conventional optical modulators, the modulation signal waveform deteriorates as the bit rate increases, resulting in a problem that it is difficult to generate equivalent optical signal waveforms irrespective of the bit rate.
To solve the above problem, the extinction characteristic of an optical modulator is required to have a small gradient near the 1-level. The extinction characteristic herein means an optical output power vs. application voltage (V) characteristic. A description will be made below by using extinction characteristics of conventional optical modulators shown in FIGS. 10A to 10C.
In FIG. 10A, the horizontal axis represents the application voltage (V) and the vertical axis represents the output power of a modulated optical signal. Symbol Vpp represents a voltage amplitude (V), Vo represents a voltage aperture (V), and ΔV represents a spread (V) of the application voltage. Curves a and b are extinction characteristics of different optical modulators. FIG. 10B shows a modulated optical waveform and FIG. 10C shows a modulation signal waveform. As shown in FIG. 10A, the extinction characteristic curves a and b exhibit the same extinction ratio Ex (dB) for the same voltage amplitude Vpp. The extinction ratio means a ratio of a maximum value to a minimum value that are obtained when the transmission optical intensity is varied in an optical modulator. The term “gradient near the 1-level” (of each of the extinction characteristic curves a and b) means an extinction ratio Ex (dB) for a spread of the application voltage on the 1-level side of the modulated optical signal (on the 0-V side of the application voltage in FIG. 10A). As shown in FIG. 10A, in the extinction characteristic curve a (i.e., in the optical modulator exhibiting such an extinction characteristic), the gradient near the 1-level is small, that is, an extinction ratio Ex, a (dB) for a 1-level-side spread ΔV of the modulation signal waveform is small. On the other hand, in the optical modulator that exhibits the extinction characteristic curve b (i.e., in the optical modulator exhibiting such an extinction characteristic), the gradient near the 1-level is large, that is, an extinction ratio Ex, b (dB) for the 1-level-side spread ΔV of the modulation signal waveform is large. A relationship Ex, a<Ex, b holds. In the modulated optical waveform shown in FIG. 10B, a spread of the modulated optical waveform near the 1-level corresponds to the extinction ratio Ex, a or Ex, b. Therefore, to obtain an optical modulator that provides a small spread of a modulated optical waveform near the 1-level and hence enables a good bit error ratio, an extinction characteristic curve that is like the curve a rather than the curve b should be realized. That is, an optical modulator is necessary in which the extinction ratio Ex (dB) is kept approximately the same and the gradient near the 1-level of the extinction characteristic is kept small as the bit rate is increased.
However, a general empirical rule is such that if a means for decreasing the gradient near the 1-level of the extinction characteristic without sacrificing the extinction ratio Ex (dB) is used, for example, if a means for adjusting the well width or barrier height of multiple quantum wells (MQWs) that are used in an optical absorption layer of an optical modulator is used, the degree of chirping increases necessarily. That is, there is a problem that it is difficult to lower the degree of chirping without deteriorating the extinction characteristic of an optical modulator. It is considered that this problem prevents realization of a low-chirp modulator that can be used at such a high modulation rate as 40 Gbps. As a matter of fact, no low-chirp modulators capable of operating at 40 Gbps have not been reported yet at academic meetings etc.